The invention is directed to an optical waveguide fiber having a multi-ring core design which provides for large negative dispersion. In particular, the large negative dispersion is achieved while maintaining low bend loss, low splice loss, and polarization mode dispersion comparable to that of the waveguide fiber comprising the link to be compensated.
Many telecommunications links designed to operate in a wavelength window near 1300 nm have been installed. In general the waveguides manufactured for such links were designed to have a zero dispersion wavelength near 1300 nm to avoid signal distortion due to dispersion over long, unregenerated link lengths. More recently, the capability of operating at a wavelength window near 1550 nm has been developed. The 1550 nm window is advantageous because the attenuation of a silica based waveguide has a minimum there and the window lies near the center of the erbium doped optical amplifier gain curve. In fact, for a typical waveguide the attenuation near 1550 nm is less than 60% of the attenuation near 1300 nm. This large gain in signal to noise ratio as well as the possibility of extending link length without adding regenerators has made telecommunications operations at 1550 nm very attractive.
However, if a telecommunication link originally made for 1300 nm window operation is to be upgraded to include 1550 nm operation, the dispersion penalty due to the location of the dispersion zero must be overcome. Because the waveguides in these links weredesigned to have zero dispersion near 1300 nm, the dispersion at 1550 nm increases rapidly with link length. The dispersion at 1550 nm is typically about 15-20 ps/nm-km.
Two strategies which may be used to remove the 1550 nm dispersion penalty are:
employ very narrow line width 1550 nm lasers; or, PA1 introduce dispersion compensating waveguide fiber into the link. PA1 is resistant to bending loss; PA1 has a high negative dispersion so that the compensating fiber length is relatively short, PA1 has a low attenuation; PA1 exhibits low splice loss with the original system waveguide; and, PA1 has comparatively low polarization mode dispersion.
A dispersion compensating waveguide fiber is one which has a dispersion of opposite sign relative to the link dispersion which is to be compensated. For example, a 1300 nm telecommunications system may have 18 ps/nm-km dispersion at 1550 nm. A link length of 60 km is common. Thus, for 1550 nm operation over this link 1080 ps per nm of laser line width must be compensated to avoid a dispersion penalty. Even though very narrow line width distributed feedback lasers have been developed a considerable amount of dispersion remains to compensated at operating wavelengths far from the zero dispersion wavelength. Thus the dispersion compensating waveguide fiber can be used to advantage in systems employing very narrow linewidth lasers as well as lasers having a relatively broad emission band.
The technical innovations required to implement either of these strategies are sophisticated. In the case of the compensation waveguide strategy, waveguide core profiles which provide the proper amount of compensating dispersion must be found. The problem is complicated by the fact that changing the core refractive index profile to obtain a negative, i.e., compensating, dispersion, changes other properties of the waveguide. In particular, waveguide fibers having negative dispersion have been found to be more susceptible to bending loss, have higher polarization mode dispersion, and increased attenuation when compared to the original waveguide fiber used in the system. See. for example U.S. Pat. No. 5,361,319, Antos, et al.
The telecommunications industry, therefore, has a need for a dispersion compensating waveguide fiber which: